By Stephen O’Riorden

With much of the world’s critical infrastructure aging, regular inspection is essential for ensuring the safety, integrity and longevity of these structures. Bridges, tunnels, dams, pipelines, and underwater structures all need thorough and regular inspections. When human divers face limitations due to depth, visibility or hazardous conditions, ROVs become necessary (Sea Technology, November 2024). These underwater robots have become the go-to solution for detailed, safe and efficient infrastructure inspections.

ROVs enable operators to inspect structures in deep, murky waters; monitor hard-to-reach areas; and collect data that would otherwise be difficult and dangerous to obtain. A crucial component of these ROV systems is their communication link with the operators on the surface.

The solution to maintaining strong, real-time control over ROVs in challenging underwater environments is the use of fiber-optic tethers. These cables provide a reliable, high-bandwidth connection, enabling real-time data transfer, video streaming and precise control of the vehicle.

Fiber-optic cables are central to the functionality of many underwater ROVs. Unlike wireless communication, which is limited in underwater environments due to signal attenuation, fiber optics offer a reliable, high-bandwidth communication link between the ROV and its surface operators. Linden Photonics specializes in the development of durable fiber-optic cables designed for harsh environments, making products particularly suitable for underwater applications.

Key Applications of ROVs in Infrastructure Inspection

Bridge Piers and Supports. Many bridges span bodies of water, meaning their supports and foundations are submerged. These critical components must be inspected for corrosion, cracking or scour (the erosion of soil around bridge foundations). ROVs are used to navigate around these submerged structures and visually inspect them or use sonar to detect damage that is not visible to the naked eye.

Dams and Reservoirs. Dams, which control water flow and generate hydroelectric power, have large underwater foundations and intakes that need regular inspection to prevent failures. ROVs can access these areas to detect sediment buildup, erosion or cracks in the concrete. They also monitor the condition of floodgates and intake pipes.

Offshore Structures. Oil rigs, wind farms and offshore platforms are often located in deepwater, where human divers cannot operate for long periods. ROVs can perform inspections of these structures to ensure their stability and monitor the condition of pipelines and subsea equipment.

Underwater Tunnels. Tunnels constructed under rivers or oceans, used for transport or water conveyance, need regular checks for leaks, structural integrity and obstructions. ROVs can easily navigate through these confined spaces, taking video footage and using sonar for a comprehensive assessment.

Pipelines. Subsea pipelines, which transport oil, gas or water, stretch for hundreds of miles underwater. ROVs can conduct detailed inspections to detect any signs of corrosion, cracks or leaks that could lead to environmental disasters.

Fuel Tanks. Inspecting fuel tanks with ROVs is a safer and more efficient method than manual inspections. ROVs can navigate confined spaces and assess the integrity of tank walls, detect corrosion, and check for contaminants without draining or disrupting operations. However, the caustic nature of certain fuels can be highly damaging to the fiber-optic tether used to control the ROV. Exposure to chemicals and corrosive substances can degrade the tether’s jacketing, leading to potential loss of signal transmission or even complete operational failure. To mitigate this, the tether must be made from materials resistant to chemical exposure, ensuring safe and reliable inspections. Regular maintenance and inspection of the tether itself are also crucial for sustaining performance over time.

Fiber-Optic Tethers in ROV Operations

While ROVs play an essential role in infrastructure inspection, their effectiveness largely depends on their ability to communicate with operators at the surface. Unlike above-water drones, which can rely on wireless signals, ROVs must use physical tethers to maintain communication underwater due to the high signal attenuation in water. Hence, the need for fiber-optic tethers.

Fiber-optic tethers do more than just enable basic communication between the ROV and the surface: They facilitate the use of advanced technologies and methods that make inspections more effective.

Inspections often rely on visual data to detect cracks, corrosion or material degradation. Fiber-optic tethers enable the transmission of high-definition video feeds in real time, even in low-visibility underwater environments.

In murky waters where visibility is limited, sonar imaging is often used to map structures and detect damage that is invisible to the eye. Fiber-optic tethers enable the rapid transmission of sonar data, providing inspectors with detailed 3D images of the underwater environment.

Many ROVs are equipped with manipulator arms that allow operators to physically interact with underwater structures, such as collecting samples or placing sensors. Fiber-optic tethers ensure that these manipulations can be controlled with precision, even over long distances.

Advantages of Fiber-Optic Cables

Fiber-optic cables transmit data using light, enabling much higher bandwidth compared to traditional copper cables. This is essential for ROV operations, which often involve transmitting high-resolution video feeds, sonar data, and control signals between the ROV and the surface in real time.

Unlike copper cables, fiber optics experience much less signal degradation over long distances, allowing ROVs to operate far from the deployment vessel. This is critical for inspecting large infrastructure, such as bridges and tunnels that may extend over hundreds of meters underwater.

Linden Photonics has developed fiber-optic cables that are not only thin and lightweight but also highly durable and resistant to mechanical stress, water ingress, and other environmental factors. This is particularly important in underwater settings, where cables can be subjected to harsh conditions, such as high pressure, currents and abrasion.

Linden Photonics has developed a range of advanced fiber-optic cables designed specifically for harsh environments, including the subsea industry. Strong Tether Fiber Optic Cable (STFOC) and its buoyant variant (BSTFOC) are built for underwater ROV applications. These cables combine high strength and durability with a compact, lightweight form factor and varying levels of buoyancy.

Cables can be reinforced with materials such as Kevlar, which provides significant tensile strength while maintaining flexibility. This allows the cable to withstand the forces exerted by underwater currents and the ROV’s own movements. Cables are designed to minimize optical loss under high pressures, which is essential for deepwater operations.

The outer layers of the cables are constructed from materials that resist wear and tear from contact with rocks, debris, and other underwater hazards.

 

Case Study: Francis Scott Key Bridge Inspection

After the Francis Scott Key Bridge collapsed in Baltimore, Maryland, on March 26, 2024, Carl Shipley, head of the U.S. Coast Guard’s Remotely Operated Vehicle and Underwater Port Security (ROV & UPSEC) program, was called to Baltimore Harbor to assist in the search for victims. Using the Coast Guard’s Strategic Robotic Systems (SRS) Fusion ROV, Shipley and his team identified two vehicles within the wreckage and provided their GPS coordinates to dive teams by the end of the day.

However, three weeks later, one victim was still missing. When Maryland State Police requested further assistance, Shipley decided to try a new approach, inspired by techniques developed after the Lahaina, Hawaii, wildfires. The plan involved integrating short-range unmanned aircraft systems (SR-UAS) with the ROV to create a real-time map of the area, improving underwater target identification, though this method had never been field-tested.

Petty Officer First-Class Claudio Giugliano, an SRUAS pilot with the Atlantic Strike Team, began by capturing aerial imagery of the bridge using a short-range drone. Shipley then turned these images into an orthomosaic map, overlaying real-time data onto the ROV’s navigation system. This updated map was far more accurate than Google satellite images, which were outdated and less effective in identifying submerged objects, especially in changing environments like the collapsed bridge.

The combination of new aerial data and side scan sonar operations, led by Shipley and Petty Officer First-Class Miguel DeJesus-Vega, mapped over 317,000 sq. yards of the seafloor and enabled the team to identify remaining debris. This approach helped them narrow down search areas and determine that the missing victim was likely still trapped in the debris, requiring careful recovery efforts before starting demolitions.

Shipley believes this integrated approach will be valuable for future Coast Guard operations, including search and rescue, marine environmental protection, and disaster relief efforts. The ability to coordinate aerial and underwater systems provides a clearer operational picture in disaster recovery and port reopening efforts.

“We now have the capability to assess above and below the surface,” Shipley said.

The successful use of Linden Photonics fiber-optic cables in the Francis Scott Key Bridge inspection underscores the broader potential of this technology for infrastructure inspection. As more of the world’s infrastructure ages, the demand for reliable, noninvasive inspection techniques will only increase.

Case Study: A Very Long Tunnel

While the Francis Scott Key Bridge inspection was conducted in a river environment, the same principles apply to other underwater infrastructure inspections, such as tunnels and pipelines.

A new world record for underwater tunnel inspections was set by Hibbard Inshore at 15.9 km (9.9 mi.) using a Saab AUV/ROV and a single continuous Linden fiber-optic cable. (The previous record was 14.1 km.) The inspection of the newly built tunnel provided the client with comprehensive data, including georeferenced 3D point clouds, sonar mosaics and other information, which can be compared to future inspections for monitoring over time.

Before starting the inspection, rebar was cut and removed at a depth of 500 m by a single ROV to clear the way for the inspection vehicle, which was an AUV/ROV that had to navigate three 90° bends before entering the main tunnel to successfully complete the inspection.

The vehicle has the capability to travel even farther distances for future projects. The team is now looking for the next opportunity to set a new world record for underwater tunnel inspections.

Conclusion

As fiber-optic technology continues to evolve, ROVs equipped with advanced cables will be able to handle even more complex tasks. For example, future advancements in fiber-optic sensors could enable ROVs to gather even more detailed data, such as real-time measurements of pressure.

The inspection in the aftermath of the Francis Scott Key Bridge collapse using the SRS Fusion ROV and Linden Photonics fiber-optic cables demonstrates the significant advantages of this technology in mission-critical underwater infrastructure inspection. Fiber-optic cables enable long-distance, real-time data transmission, ensuring that ROVs can thoroughly inspect complex underwater structures in challenging conditions. Moreover, the durability and flexibility of the cables ensure reliable performance throughout a mission.

As infrastructure continues to age and the need for safe, efficient inspection methods grows, fiber-optic cables will play an increasingly important role in enhancing the capabilities of underwater ROVs. Linden Photonics’ innovative cable technology is well-positioned to meet the demands of this evolving field, providing the reliability, performance and durability needed for critical infrastructure inspections around the world.

Stephen O’Riorden is the president of Linden Photonics.